Counterbalancing systems for sectional doors have been employed for many years. Common examples of such sectional doors are the type employed as garage doors in homes, commercial and utility buildings, and similar applications. Counterbalancing systems originally solved the need for providing mechanical assistance in the instance of very large doors for commercial installations and smaller garage doors for residential use, which were normally constructed of heavy, relatively thick wood or metal components. More recently, counterbalancing systems have been increasingly used to permit opening and closing operations by a single person and to facilitate the use of electric motors, preferably of limited size, to power the opening and closing of such doors.
Most such counterbalancing systems utilize drums which carry cables attached to the garage door. Commonly the drums are mounted above the frame defining the door opening, with a drum positioned at each end of the door such that the cables may be conveniently connected proximate the lower lateral corners of the garage door. Basically, the door is moved toward the closed position, blocking the door opening due to gravity acting on the door as it moves from a substantially horizontal, open position above and inwardly of the door frame to a closed position. The path of the door in opening and closing is commonly defined by a track arrangement which interacts with rollers attached to the various sections of the door. The cable drums are classically interconnected with springs in a wide variety of ways so that they are progressively loaded as the door is lowered to prevent uncontrolled descent of the door and employ stored energy to assist in raising the door during subsequent opening operation.
The prevailing type of counterbalancing system for garage doors for homes normally having a seven-foot high door involves the utilization of torsion springs mounted on a shaft which is coaxial with or mounts the drums. In such systems, it is established practice to utilize cable drums having a diameter of approximately 31/2 inches to 4 inches. A torsion spring or springs mounted outwardly of the shaft has a diameter normally in excess of 11/2 inches to maintain an appropriate spring index. The drums and spring are normally mounted on a tubular shaft having a diameter of approximately 1 inch, which holds the springs and transmits torque from the springs to the drums which are attached to the tubing.
These conventional torsion counterbalancing systems require that the tube mounting the drums be positioned above the horizontal track of the door to permit raising the door as high in the door opening as possible to accommodate higher vehicles and to otherwise make optimum use of the door opening. With a counterbalancing system thus positioned and employing conventional 31/2 to 4-inch cable drums, there is a requirement that there be a minimum of 13 to 14 inches above the door opening as overhead clearance to permit the mounting of these counterbalancing systems. However, a disadvantage of these conventional systems is the increasing requirement for a counterbalancing system which can be installed in a structure having a lesser overhead clearance. Frequently, construction parameters dictate a lower ceiling within a garage or the use of beams, supports, or other objects which do not provide the necessary headroom clearance of 13 to 14 inches required for the utilization of these conventional counterbalancing systems.
In an attempt to accommodate the requirements for decreased overhead clearance, efforts have been made to modify these conventional counterbalancing systems. If the drums and tube with the mounted springs are merely moved downwardly, one or more of these elements interfere with the door during its opening and closing motion. One alternative which has been employed to solve reduced headroom requirements is to move the drums outboard or laterally of the tracks and lowered to a point that the springs and center bracket supporting the tube normally substantially medially thereof will just permit door clearance. This configuration, however, has serious limitations in that the cable binds the door to some extent due to the outward force applied during operation, and such is only effective to minimally reduce headroom clearance to a distance on the order of 12 inches; however, this expedient tends to increase the required clearance distance to the sides of the door frame.
Another approach to meeting low headroom requirements is a reversion to the use of one-piece door systems. These systems, which may or may not employ track systems, normally pivot the doors about a point approximately vertically medially of the door opening. One-piece door systems have not achieved a substantial acceptance due to one or more of a combination of disadvantages. These systems require assured clearance either inside or outside the door anytime it is opened or closed, depending on whether the door swings inwardly or outwardly, respectively. Normally these systems require additional side clearance to accommodate the pivoting mechanism and the counterbalance system. Finally, large one-piece doors are essentially prohibitive, and even small doors are highly disadvantageous in terms of packaging, shipping, transporting, and installing the doors.
A more drastic alternative to obtain additional headroom contemplates the movement of the entire counterbalance system to the rear of the horizontal track, i.e., inwardly of the garage to a position proximate the extremities of the horizontal track where the top of the door reposes when it is in the open position. In systems of this nature, it is necessary to route the cable by pulleys from the counterbalance system to the door frame and then to the door. Systems of this type have proven to be both inefficient and costly, while introducing a relatively large, unsightly mechanism centrally of a garage. In addition, such systems often result in a geometry such that the lower portion of the bottom panel of the door does not reach the lower edge of the header but rather hangs down a substantial distance into the door opening when in the horizontal, open position.
This hang-down characteristic is particularly critical in the case of below level garages where the driveway angles downwardly to the door, such that a vehicle is in an angular raised position when passing through the door opening. Hang-down in existing door operations systems results from a combination of factors. Initially, a properly counterbalanced door has the spring tension approaching zero as the door moves from the closed to the open position and the weight of the door is progressively transferred from the cables of the counterbalance system to the horizontal section of the track system. Further, the guide roller proximate the bottom of the door is located above the bottom of the door and on the inside surface such that it is located in the curved transitional track section when the door is in the fully open position. The cable that is routed from the track system proximate the door opening is at a substantial increasing angle to the direction of travel of the door as the bottom roller moves increasingly into the curved transitional track section. Thus, in nearing the open position of the door, the force component operative to further open the door is reduced by the diminishing spring tension force and its increasing angle of application to the door. As a result, a substantial hang-down of doors into the door opening is common and may even require a door having a greater vertical height than would otherwise be required in some applications.
If a conventional counterbalance system is rear-mounted in a low headroom environment, a substantial portion of the system normally extends a distance below the horizontal track section. This configuration produces dangerous and thus undesirable conditions. First, the counterbalance springs are totally exposed to a person in the garage rather than being against the header where the door is between the springs and the person during most of the operating sequence. Second, persons of even average height may be exposed to the possibility of head injury and the irritation of interference with objects being carried in a garage having such an installation.
Rear-mounted counterbalance systems in low-overhead environments where it is necessary to maintain the horizontal track sections at the lowest possible height above the door header often experience difficulty in seating and locking the top door panel against the header in manually-operated door installations. In particular, linear or slightly curved tracks proximate the header may operate to effect closing and opening; however, in such installations even minimal forces, e g., wind, applied to the outside of the top panel can result in its unseating and uncontrolled opening. In many instances, prior-art systems have endeavored to solve low-overhead environments by displacing one or more components of the counterbalance system laterally outwardly of the tracks. However, in many instances, there are complexity and performance sacrifices created, and, in some instances, no solution is realized because low-headroom conditions are not infrequently accompanied by minimal side room to one or both sides of a door opening.
For example, the cable drums may be moved outside the track to preclude interference with the door; however, this is possible only where there is substantial clearance on both sides of a door and any adjacent wall or other obstruction. Other systems place the counterbalance drums inside of the rear ends of the horizontal track sections and route the cable over the horizontal track section and along the outside of the vertical track section to the bottom of the door. In these installations, the drum must be positioned above the track a sufficient distance to preclude the cables from abrading on the horizontal track sections, thereby requiring additional overhead clearance. In the instance of either of the above-described outside or inside drum mounting, the cable may interfere in the vertical cable run with photo eye sensors that are now required for radio-controlled motor-operated doors.
The aforedescribed conventional torsion spring counterbalancing systems also have the disadvantage that the weight of the spring members is such as to require the use of a support bracket which normally suspends the tubular shaft substantially medially between the drums. The stationary support bracket is also commonly employed as the stationary anchor for the torsion springs. The support bracket is attached to the door header or more commonly a special spring pad located on the garage wall thereabove. Since the stationary anchor associated with the support bracket undergoes torsional loading equal to the weight of the door, there is a constant potential for operational failure or damage and injury to installation and maintenance personnel. The torsional forces can also result in a loosening of the support bracket, loosening of the stationary spring anchor, a failure of a door opening header or spring pad, all of which can result in a quick and violent untensioning of torsion springs, thereby presenting the potential for damage or injury to any proximate objects. In the case of a conventional rearmount counterbalance system, the tensioning of conventional torsion springs in a low-headroom environment is very difficult because of the lack of clearance to manipulate the tensioning bars. Further, the center support bracket must be adequately supported in a cantilevered position due to the torsional loading imparted by the springs even at the expense of additional time or material.
Another disadvantage of such conventional torsion spring counterbalancing systems is the susceptibility to variations in balance of the door. With a drum diameter of approximately 4 inches, the drums revolve approximately seven times during an opening cycle of a 7-foot high door. As spring tension is lost through aging or extensive use, a highly noticeable variation in balance of the door is produced, as contrasted with systems which might have a lesser drum diameter and, therefore, rotate a greater number of times during opening and closing, such that the loading effect on a door is less for a given variation in spring tension. This same consideration makes it difficult to adjust the conventional 4-inch drum systems, since minute adjustments in spring tension can produce a substantial effect on a door.